This invention relates in general to chairs and more particularly, to wheelchairs. Most particularly, the invention relates to wheelchair seat backs that are movable and flexible to accommodate an increased load capacity.
The anatomy and biomechanics of the human spine with normal neuromuscular function could be described as having an anterior curve in the lumbar area, a posterior curve in the thoracic area, and an anterior curve in the cervical area. When a person is in a seated position, large muscle groups, for example, the abdominal muscles and the spinal extensors, work hard in harmony to hold the body in a state of balance. The pelvis provides a support foundation, upon which the spine and the head are balanced.
When a person is in a seated position, the pelvis needs to be neutral or in a slightly anterior position in order for the spine and head to be in their most stable and therefore functional position. Three conditions affect the needs of the pelvis when a person is seated in a conventional seat.
First, the pelvis encounters posterior tilt or rocks backwards. When the pelvis rocks backwards, the spinal curves change. For example, the thoracic spine curvature increases, or becomes kyphotic, and the lumbar spine flattens or loses its anterior curve. This is not a desirable position for safety, function or skin protection. Therefore, it is desirable to block the posterior pelvis so as to prevent posterior rocking of the pelvis, which will happen if the pelvis is unsupported due to the effects of gravity and fatigue of the major muscle groups.
Second, the back tends to flatten. The thoracic spine has a natural posterior curve. A flat back does not support a functional posture. Moreover, it causes fatigue. To prevent the muscles from having to work too hard and ultimately fatiguing, the spine needs to be supported accordingly.
Third, the gluteal mass or soft tissue tends to spread in a posterior curve below a hinge point of the seat back. When unaccommodated by a back support, this causes the person to slide forward in the seat and consequently lose posterior pelvic contact with the seat back, which further causes undesirable posterior pelvic tilt.
What is needed is a seat back assembly that will solve the above-identified problems by blocking the posterior pelvis at the level of the anatomic hinge point in the spine with a back support hinge that lines up with the anatomic hinge. The seat back assembly should extend posteriorly above the hinge point to accommodate the natural curvature and biomechanics of the spine. Moreover, it should flare posteriorly beneath the hinge point to accommodate the curvature of the gluteal mass or soft tissue.
The present invention is directed towards a seat back assembly for connecting a seat back shell to the seat back tubes of a wheelchair. The seat back assembly comprises a shell, upper and lower shell connectors, and upper and lower side plate portions. The lower shell connector is mounted to a lower portion of the shell. The upper shell connector is mounted to an upper portion of the shell. The upper side plate portion is adapted to be mounted to an upper portion of the seat back tube. The lower side plate portion is adapted to be mounted to a lower portion of the seat back tube at a position substantially co-linear to the posterior superior illiac spine (PSIS) of a user. The lower shell connector and the lower side plate portion are pivotally engageable with one another along a pivot axis. The upper shell connector and the upper side plate portion are attachable relative to one another at discrete locations so as to permit the angular disposition of the shell to be adjusted.
Another embodiment of the invention is directed towards a wheelchair comprising a seat back tube and a seat back assembly. The seat back assembly comprises a shell, a lower shell connector, and upper shell connector and a side plate. The lower shell connector is mounted to a lower portion of the shell. The upper shell connector is mounted to an upper portion of the shell. The side plate comprises an upper side plate portion and a lower side plate portion. The upper side plate portion is adapted to be mounted to an upper portion of the seat back tube. The lower side plate portion adapted to be mounted to a lower portion of the seat back tube at a position substantially co-linear to the PSIS of a user. The lower shell connector and the lower side plate portion are pivotally engageable with one another. The upper shell connector and the upper side plate portion are attachable relative to one another at discrete locations so as to permit the angular disposition of the shell to be adjusted.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.